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PISS (Pilot Induced Speed Shimmy)

Andrew Anunson
 

I was up flying this past Sunday in my Cozy MKIV with buddy, we were at front seat weight of 390 and a CG of 99.3.
Final landing for the day, did my GUMPS check and watched the electric nose gear extend through the little plans built windows in the wheel well.

Good approach to landing, planted the mains, and the nose gear touched firmly.  The nose wheel came back up off the runway just a bit, and once back down a nose shimmy started.  It sounded like driving on the highway rumble strips and felt like a flat tire/rumble strip combination.  Tried to keep weight off the nose gear using back pressure on the stick but it didn't come up again, we gently slowed and the shimmy stopped at perhaps 45 kias.

I said something like "Man, that was a nosewheel shimmy!  WTF?"  I looked down at the panel, and my nosegear switch was in the middle (off) position (no longer in the down/extend position). We taxied to the hangar and did a post flight inspection.  I don't know how the switch got to that position.

I haven't ever noticed my NG mechanism back driving, but I never taxi, takeoff, or land with it in the off position.  I have an old Wilhelmson unit with the manual backup system and it doesn't have the electric brake system.  I have installed small belleville washers per Jack's instructions, and it never (noticeably) back drives.

I found the nose gear leg slightly loose in the MKNG6.  The flox bond had broke loose.  Just a slight wiggle but it needed to be re-done.
It took about 15 minutes to get the nose gear leg out, and less than an hour to clean it up and reflox the assembly at home.  

Cause:  Perhaps the nose gear switch in the "OFF" position
Effect:  Shimmy, and loss of the flox bond at MKNG6

Prevention:  Any ideas?

I have a Nose Gear warning system that alerts me when landing and the nose gear is up.  I need to see if it alerts me when I'm landing and the switch is in the OFF position.  If not, I'll change it so that it does.

NOTES:  
1.)  My nose tire did very well.  No damage.
2.)  The nose fork and attachment did very well.  I attached my nose fork to the gear leg using the "bulb and clamp method".  Thats what I call it.  I think Marc Zetilin recommended it.
3.)  Check closely for damage after anything unusual like this.

Andrew Anunson
Cozy MKIV #1273
Pound, VA







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Mike Satchell
 

Assuming that the nose gear wasn't fully extended was the cause, maybe an audible warning that the gear is not fully extended would have helped?

I tapped the yellow wire that goes to the little panel LEDs, connected it to a voltage divider circuit that feeds into the Dynon EMS220 so that the nose gear up was one voltage and nose gear extended was another (0-5 volts), then created a widget on the Dynon Skyview panel using the "gear" label.  The Skyview logic will warn audibly if the gear is extended above whatever speed you set, or will warn that the gear is not extended when the speed is low.  The gear down widget is green and gear retracted is yellow.  I hadn't looked for it, but the EFIS probably records this status too.

HTH,
Mike Satchell

On Fri, Mar 10, 2023 at 12:47 PM Andrew Anunson via groups.io <macleodm3=yahoo.com@groups.io> wrote:
I was up flying this past Sunday in my Cozy MKIV with buddy, we were at front seat weight of 390 and a CG of 99.3.
Final landing for the day, did my GUMPS check and watched the electric nose gear extend through the little plans built windows in the wheel well.

Good approach to landing, planted the mains, and the nose gear touched firmly.  The nose wheel came back up off the runway just a bit, and once back down a nose shimmy started.  It sounded like driving on the highway rumble strips and felt like a flat tire/rumble strip combination.  Tried to keep weight off the nose gear using back pressure on the stick but it didn't come up again, we gently slowed and the shimmy stopped at perhaps 45 kias.

I said something like "Man, that was a nosewheel shimmy!  WTF?"  I looked down at the panel, and my nosegear switch was in the middle (off) position (no longer in the down/extend position). We taxied to the hangar and did a post flight inspection.  I don't know how the switch got to that position.

I haven't ever noticed my NG mechanism back driving, but I never taxi, takeoff, or land with it in the off position.  I have an old Wilhelmson unit with the manual backup system and it doesn't have the electric brake system.  I have installed small belleville washers per Jack's instructions, and it never (noticeably) back drives.

I found the nose gear leg slightly loose in the MKNG6.  The flox bond had broke loose.  Just a slight wiggle but it needed to be re-done.
It took about 15 minutes to get the nose gear leg out, and less than an hour to clean it up and reflox the assembly at home.  

Cause:  Perhaps the nose gear switch in the "OFF" position
Effect:  Shimmy, and loss of the flox bond at MKNG6

Prevention:  Any ideas?

I have a Nose Gear warning system that alerts me when landing and the nose gear is up.  I need to see if it alerts me when I'm landing and the switch is in the OFF position.  If not, I'll change it so that it does.

NOTES:  
1.)  My nose tire did very well.  No damage.
2.)  The nose fork and attachment did very well.  I attached my nose fork to the gear leg using the "bulb and clamp method".  Thats what I call it.  I think Marc Zetilin recommended it.
3.)  Check closely for damage after anything unusual like this.

Andrew Anunson
Cozy MKIV #1273
Pound, VA
KEN4ZZ
 

Hi Andrew-

I don't know what you have - besides the window - to get any gear locked down indication in your cockpit.  A green gear down light is good, but a green gear down AND LOCKED light is better.  The latter signal would be triggered by a properly placed microswitch on whatever the MK IV uses (or part that must be over center in the proper place) for a positive down lock.  

The fact that you found your gear switch in the "off" position *might* indicate that your NG was almost fully down, but not quite.  Who knows when it got moved from the extend to the off position?  Shimmy is one of those resonant things that needs just the correct wrong stimulation to rear its ugly head.  If - admittedly a question mark - your NG was just a few degrees shy of full extension, that different angular condition combined with the light loading of the second NG touch down just might have been the "right" wrong condition to set off the shimmy you experienced.

Or not.

Ken

On 3/10/2023 12:47 PM, Andrew Anunson via groups.io wrote:

I was up flying this past Sunday in my Cozy MKIV with buddy, we were at front seat weight of 390 and a CG of 99.3.
Final landing for the day, did my GUMPS check and watched the electric nose gear extend through the little plans built windows in the wheel well.

Good approach to landing, planted the mains, and the nose gear touched firmly.  The nose wheel came back up off the runway just a bit, and once back down a nose shimmy started.  It sounded like driving on the highway rumble strips and felt like a flat tire/rumble strip combination.  Tried to keep weight off the nose gear using back pressure on the stick but it didn't come up again, we gently slowed and the shimmy stopped at perhaps 45 kias.

I said something like "Man, that was a nosewheel shimmy!  WTF?"  I looked down at the panel, and my nosegear switch was in the middle (off) position (no longer in the down/extend position). We taxied to the hangar and did a post flight inspection.  I don't know how the switch got to that position.

I haven't ever noticed my NG mechanism back driving, but I never taxi, takeoff, or land with it in the off position.  I have an old Wilhelmson unit with the manual backup system and it doesn't have the electric brake system.  I have installed small belleville washers per Jack's instructions, and it never (noticeably) back drives.

I found the nose gear leg slightly loose in the MKNG6.  The flox bond had broke loose.  Just a slight wiggle but it needed to be re-done.
It took about 15 minutes to get the nose gear leg out, and less than an hour to clean it up and reflox the assembly at home.  

Cause:  Perhaps the nose gear switch in the "OFF" position
Effect:  Shimmy, and loss of the flox bond at MKNG6

Prevention:  Any ideas?

I have a Nose Gear warning system that alerts me when landing and the nose gear is up.  I need to see if it alerts me when I'm landing and the switch is in the OFF position.  If not, I'll change it so that it does.

NOTES:  
1.)  My nose tire did very well.  No damage.
2.)  The nose fork and attachment did very well.  I attached my nose fork to the gear leg using the "bulb and clamp method".  Thats what I call it.  I think Marc Zetilin recommended it.
3.)  Check closely for damage after anything unusual like this.

Andrew Anunson
Cozy MKIV #1273
Pound, VA

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